Optical disks including DVDs and CDs are widely used as media for recording music, pictures, data information etc. Reproduction and recording of information from/to the media are carried out by optical pickups. Optical pickups are under development to be further miniaturized and improved in performance.
An optical pickup includes an optical receiver for receiving reflection light from optical disks. Recent optical receivers tend to include less number of components due to cost reduction and use more speedy transmission signals, and therefore the receivable amount of signal light quantity of the optical receiver is decreasing. Therefore, noise reduction is important and indispensable factor in the development of optical receivers.
Further, with the recent popularization of DVDs, optical pickups compatible both with DVD and CD are under development in order to reduce cost for optical pickups. This kind of optical pickups is required to process both laser light for DVD 650 nm in wavelength, and laser light for CD 780 nm in wavelength. Therefore, noise reduction is also an indispensable factor for the optical receiver used for such a pickup.
In an optical pickup device compatible with writing discs like CD±R, CD±RW, DVD±R, DVD±RW which are now widely used, signal reproduction and signal recording (writing) are carried out by different laser powers, and the signal light quantity emitted to the optical receiver varies. In this view, the optical pickup device normally switches the gain of photoreceiver device according to the condition of reproduction/recording. Noise reduction is also an important factor for the optical receiver used for such a pickup.
FIG. 16 shows structures of photodiodes PDA to PDF serving as optical receivers of an optical pickup compatible with a single wavelength. FIG. 17 shows a structure of photoreceiver/amplifier circuits 101a to 101d respectively including the photodiodes PDA to PDE. FIG. 18 shows a detailed structure of the photoreceiver/amplifier circuit 101a. 
Based on FIGS. 16 and 18, the following explains a basic operation principle of the photoreceiver/amplifier circuits 101a to 101d, an offset voltage characteristic which is their main characteristic, and a noise reduction method for an amplifier circuit.
As shown in FIG. 16, the square-shaped photodiodes PDA to PDD are adjacently provided to generate RF signals and focus error signals. The rectangle photodiodes PDF and PDE are provided on both sides of the photodiodes PDA to PDD to generate tracking error signals. The photodiodes PDF and PDE outputs signal voltages Ve and Vf, respectively.
The photodiodes PDA to PDD are each connected to a differential amplifier circuit AMP101 of a corresponding one of the photoreceiver/amplifier circuits 101a to 101d. Signal voltages VA to VD outputted from the photoreceiver/amplifier circuits 101a to 101d are processed according to the following calculations by a processor IC in a following stage, to become an RF signal, a focus error signal and a tracking error signal. Therefore, it is important to ensure accuracy of voltage outputs among the photoreceiver/amplifier circuits 101a to 101d. As to the signal voltages VA to VD, it is important to ensure the S/N characteristic as they are used for generation of RF signals.RF signal=Va+Vb+Vc+Vd  (1)Focus error signal=Va+Vc−(Vb+Vd)  (2)Tracking error signal=Ve−Vf  (3)
The following describes the photoreceiver/amplifier circuit 101a including the photodiode PDA to explain one example of operation principle of signal current-voltage conversion.
A signal beam is emitted to the photodiode PDA, and a photocurrent Ia is generated. The photocurrent Ia is converted to a voltage by a gain resistor R1 photoreceiver/amplifier circuit 101a, and appears in an output terminal as an output voltage Von with respect to signal beam emission. Assuming that the output voltage appearing in an output terminal under no signal emission condition is expressed as Vod, a signal voltage Vsig generated by a signal incident on the photodiode PDA is denoted by the following formula.Vsig=Von−Vod  (4)
The output Von is expressed as follows,Von=R1×Ia+Vref  (5)
where Ia expresses photocurrent, and R1 expresses gain resistance.
Vref is a reference power supply voltage, which is externally supplied.
The signal Vod is expressed as follows,Vod=Vref−R2×Ib2−VBE2+VBE1+R1×Ib1  (6)
where Ib1 and Ib2 express base currents of transistors Tr101 and Tr102 under no signal input condition, VBE1 and VBE2 express respective voltages between bass-emitter of Tr101 and Tr102, and R2 expresses a resistance of a reference resistor R2.
Accordingly, the signal voltage Vsig is expressed as follows.Vsig=R1×Ia−(−R2×Ib2−VBE2+VBE1+R1×Ib1)  (7)
In Formula (7), the second term and later is an error.
In the photoreceiver/amplifier circuit, it is important to ensure high accuracy of processing signal of the focus signal denoted by Formula (2), and therefore a decrease in accuracy of processing signal due to the error (offset voltage) becomes an issue. More specifically, it is important to satisfy: Vod≈Vref, which is met by satisfying the following Formulas (8) and (9) under no signal emission condition.R1×Ib1≈R2×Ib2  (8)VBE1≈VBE2  (9)
Assuming that an error of the output voltage Vod with respect to the external reference voltage Vref under no signal emission condition is expressed as an offset voltage Voff, the voltage Voff is denoted by the following Formula (10).Voff=Vod−Vref  (10)
The offset voltage in the photoreceiver/amplifier circuit has to fall within an accuracy level of ±15 mV. To meet this requirement, according to Formula (8), it is necessary to equalize the resistor of the reference resistor R2 and the resistor of the gain resistor R1 in the conventional photoreceiver/amplifier circuit.
Suppression of noise characteristic is generally an important factor in the photoreceiver/amplifier circuit. The “noise” here indicates noise under no signal input condition, which is generated in components constituting the circuit, mainly in the transistor and the resistor. When a voltage signal resulted from a signal beam is sufficiently large in the photoreceiver/amplifier circuit, the difference between the signal voltage and the noise level is large, in other words, a desirable S/N characteristic is ensured. However, as described, with the recent decrease in signal light quantity, noise reduction in the receiver circuit is indispensable.
With reference to the photoreceiver/amplifier circuit 101a of FIG. 18, the following explains a method of reducing the noise in the photoreceiver/amplifier circuit, which is an important factor of a photoreceiver/amplifier circuit.
An output noise level Vn of the photoreceiver/amplifier circuit 101a is expressed as follows,Vn=√{square root over ( )}(Ni12+Ni22+Nr12+Nr22)  (11)
where Ni1 and Ni2 indicate noise caused by shot-noise generated in the transistors Tr101 and Tr102, and Nr1 and Nr2 indicate noise caused by thermal noise generated in the resistors R1 and R2. Vn is a square mean value of the noise generated in each component. If the bias currents of transistors Tr101 and Tr102 are greatly reduced, the shot-noise of the Ni1 and Ni2 will be reduced. However, to ensure the response characteristic of the photoreceiver/amplifier circuit 101a, the driving current cannot be extremely reduced. Further, the gain resistor R1 cannot be reduced, as it is required to have a predetermined resistance. The only way of reducing noise is setting only the resistance value of the reference resistor R2 to meet a relation: R1>>R2. However, as described, the offset voltage characteristic deteriorates in the conventional photoreceiver/amplifier circuit 101a, and therefore the relation R1=R2 has to be met.
Some prior art documents, such as Japanese Laid-Open patent application Tokukaihei 11-296892 (published on Oct. 29, 1999), discloses a structures in which extra thermal noise is generated by the reference resistor R2 in the optical receiver circuit 101a. The thermal noise is expressed as √(4kTR2Δf), where k expresses Boltzmann constant, T expresses absolute temperature, and Δf expresses noise bandwidth. This publication teaches a method of integrating the thermal noise by a resistor and a capacitor so as to reduce the thermal noise. The photoreceiver/amplifier circuit 101a includes a capacitor C1 between a node of the reference resistor R2 and the base of the transistor Tr102 and the GND, so as to provide a fixed potential (GND potential) to the reference resistor R2. With such a capacitor C1, the reference resistor R2 and the capacitor C1 form a filter for integrating noise, and radio-frequency noise can be reduced.
FIG. 19 shows structures of photodiodes PDA to PDF, and PDa to PDd used as optical receivers for the optical pickup compatible with two wavelengths. FIG. 20 shows structures of photoreceiver/amplifier circuit 101A to 101D including the photodiodes PDA to PDF, and PDa to PDd. Further, FIG. 21 shows a detailed structure of the photoreceiver/amplifier circuit 101A.
As shown in FIG. 19, the photodiode PDA to PDD, and photodiode PDa to PDd are adjacently provided between the photodiodes PDE and PDF, along the longitudinal direction of the photodiodes PDE and PDF. The photodiodes PDA to PDD correspond to the first wavelength (eg. CD), and the photodiodes PDa to PDd correspond to the second wavelength (eg. DVD).
As shown in FIGS. 20 and 21 (showing only the photodiodes PDA and PDa), the photodiodes PDA to PDD are each connected to a differential amplifier circuit AMP111 of a corresponding one of the photoreceiver/amplifier circuits 101A to 101D, and the photodiodes PDa to PDd are also each connected to a differential amplifier circuit AMP111 of a corresponding one of the photoreceiver/amplifier circuits 101A to 101D. The differential amplifier circuit AMP111 includes a differential circuit DEF1 including transistors Tr101 and Tr102 and a bias circuit I1; and a differential circuit DEF2 including transistors Tr103 and Tr104 and a bias circuit I2. In the differential circuit DEF1, the gain resistor R1, the reference resistor R2 and the photodiode PDA are connected. In the differential circuit DEF2, the gain resistor R3, the reference resistor R4, and the photodiode PDa are connected. The active load circuit AL is a common circuit for the two differential circuits DEF1 and DEF2.
The capacitor C1 is connected between a node of the reference resistor R2 and the base of the transistor Tr102 and the GND, so as to provide a fixed potential (GND potential) to the reference resistor R2. Meanwhile, The capacitor C2 is connected between a node of the reference resistor R4 and the base of the transistor Tr104 and the GND, so as to provide a fixed potential (GND potential) to the reference resistor R4.
Further, the operation of one of the bias circuits I1 and I2 in the differential circuits DEF1 and DEF2 is controlled by a selecting signal SEL externally supplied. In this manner, the operation of one of the differential circuits DEF1 and DEF2 is controlled according to the wavelength of input signal. Also, the photodiode PDA, the gain resistor R1 and reference resistor R2, the photodiode PPDa, the gain resistor R3 and the reference resistor R4 are selectively brought into operation according to which of the differential circuit DEF1 and the DEF2 is in operation.
FIG. 22 shows structures of photoreceiver/amplifier circuit 101AA to 101DD used in an optical pickup which uses photodiodes PDA to PDF of FIG. 16 as optical receivers compatible with 1 wavelength and two kinds of signal light quantity on reproduction and recording. FIG. 23 shows a detailed structure of the photoreceiver/amplifier circuit 101AA.
As shown in FIGS. 22 and 23 (showing only the photodiode PDA), photodiodes PDA to PDD are each connected to a differential amplifier circuit AMP121 of a corresponding one of the photoreceiver/amplifier circuits 101AA to 101DD. As with the differential amplifier circuit AMP101, the differential amplifier circuits AMP121 includes a differential circuit having the transistors Tr101 and Tr102. Also, in addition to the gain resistor R1 and the reference resistor R2, the differential amplifier circuit AMP121 has a gain resistor R5 and a reference resistor R6, which are provided in parallel with the gain resistor R1 and the reference resistor R2, respectively. The gain resistor R5 is conducted in parallel to the gain resistor R1 when the switch SW101 formed of a PNP transistor is turned on. The reference resistor R6 is conducted in parallel to the gain resistor R2 when the switch SW102 formed of a PNP transistor is turned on.
Assuming that the resistances of the gain resistors R1 and R5 are expressed as R1 and R5, respectively, the relationship between the two resistances is expressed as follows.R1>R5  (12)
On reproduction of signals, the signal light quantity is small and a high gain is required. The switch SW101 is therefore turned off. Consequently, only the gain resistor R1 is driven, and the reproduction signal undergoes I-V conversion. On recording of signals, the signal light quantity is large and a low gain is required to prevent circuit saturation caused by a signal of large light quantity. Therefore, the SW101 is turned on when signals are written, and the gain resistors R1 and R5 are driven. The gain Gon at this stage is expressed as follows.Gon=R1×R5/(R1+R5)  (13)
Note that, the ON resistance of a PNP transistor constituting a switch SW101 is not taken into account.
The method disclosed in the foregoing publication achieves reduction of radio-frequency noise; however, to sufficiently reduce noise in the low-frequency band, the capacitor C1 needs to have a maximum capacitance in the photoreceiver/amplifier circuit 101a of FIG. 18. This is not desirable particularly in terms of cost. In this view, there has been a demand for a noise reduction method as a replacement of the foregoing conventional technique.
Further, as described, the noise reduction is an important factor also in a photoreceiver/amplifier circuit compatible with 2 kinds of wavelength. However, as with the photoreceiver/amplifier circuit 101a of FIG. 17, the conventional photoreceiver/amplifier circuit 101A shown in FIG. 21 requires reference resistors R2 and R4 having the same resistances as those of gain resistors R1 and R3, so as to ensure a desirable offset voltage characteristic. Therefore, the photoreceiver/amplifier circuit 101A of FIG. 21 also has a difficulty in noise reduction.
The condition: R1=R2 needs to be satisfied also in the photoreceiver/amplifier circuit 101AA of FIG. 22 to ensure a desirable offset voltage characteristic. Therefore, when the gain is switched by the switch SW101, the reference resistor is also required to be switched by the switch SW102.
Considering noise in such a photoreceiver/amplifier circuit with a gain switching function, the desirable level of S/N characteristic can be ensured in the writing state because the signal light quantity is large. However, in the signal reproduction, the signal light quantity is small and the influence of noise becomes more significant, and a low noise characteristic is required. More specifically, the photoreceiver/amplifier circuit with a gain switching function needs to have a low noise characteristic according to circumstance.